At present, fine particles such as oxide fine particles, nitride fine particles and carbide fine particles have been used in the fields of production of sintered bodies for use as electrical insulation materials for semiconductor substrates, printed circuit boards and various electrical insulation parts, materials for high-hardness and high-precision machining tools such as cutting tools, dies and bearings, functional materials for grain boundary capacitors and humidity sensors, and precision sinter molding materials, production of thermal sprayed parts such as engine valves made of materials that are required to be wear-resistant at a high temperature, and production of electrode or electrolyte materials and various catalysts for fuel cells.
Among the foregoing fine particles, a reduced titanium oxide is recently drawing attention. The reduced titanium oxide is different from TiO2 (titanium dioxide), and refers to substoichiometric titanium oxides such as a Mgneli phase of TinO2n−1 [where n=4 to ∞] called a phase, a Ti3O5 phase, a Ti2O3 phase and a TiO phase.
The reduced titanium oxide is more excellent than TiO2 in not only visible light absorption but also electron conductivity, and accordingly, its applications as an electrode and a conductive filler are highly expected.
The present applicant proposes a production method of substoichiometric oxide particles that is capable of producing nanosized fine particles of high-purity substoichiometric oxide, such as a substoichiometric titanium oxide, by use of thermal plasma in a short time, in Patent Literature 1.
In Patent Literature 1, for example, when substoichiometric titanium oxide fine particles are produced, titanium oxide powder such as titanium dioxide (TiO2), and at least one of titanium metal powder, powder of a titanium compound of an element other than oxygen such as titanium hydride, and substoichiometric titanium oxide powder are supplied into thermal plasma flame separately or as a previously-prepared mixture.
In Patent Literature 1, at least two types of powders, i.e., titanium oxide powder, and at least one of titanium metal powder, titanium compound powder free of oxygen, and substoichiometric titanium oxide powder are used as raw materials. It is described that since no other metallic element than titanium constituting substoichiometric titanium oxide fine particles is used as a metallic element, high-purity substoichiometric titanium oxide fine particles can be obtained.